We are attempting to analyze the cell-cell interaction between killer T-lymphocytes and allogeneic target cells. Since it is clear that this occurs via receptors on the T lymphocyte for H-2 antigen, we have attempted to study the binding of plasma membrane vesicles bearing these antigens to the killer lymphocytes. Although specific binding of the vesicles can be detected by indirect immunofluorescence, quantitative measurements using radioactive vesicles fail to give evidence for specific binding. Furthermore, no specific inhibition of cytotoxicity was achieved by the vesicles even when aggregated into a multivalent assay. However the same amount of antigen on live "cold" target cells very efficiently blocks cytotoxicity in a specific fashion. At this stage we can only conclude that H-2 antigen alone is not adequate to establish a strong binding to the killer T cell receptor for alloantigen.